The basic image-forming process of silver halide photography comprises the exposure of a silver halide photographic recording material to actinic radiation (for example, light or X-rays), and the manifestation of a usable image by the wet, chemical processing of the material. The fundamental steps of this processing entail, first, treatment of the recording material with one or more developing agents wherein some of the silver halide is reduced to metallic silver. With black-and-white photographic materials, the metallic silver usually comprises the desired image. With color photographic materials, the useful image consists of one or more images in organic dyes produced from an oxidized developing agent formed where silver halide is reduced to metallic silver.
To obtain useful black-and-white images it is usually desirable to remove the undeveloped silver halide, and to obtain useful color images it is usually desirable to remove all of the silver from the photographic element after the image has been formed. In black-and-white photography the removal of undeveloped silver halide is accomplished by dissolving it with a silver halide solvent, commonly referred to as a fixing agent, usually in an aqueous solution called a fixer bath. In color photography the removal of silver is generally accomplished by oxidizing the metallic silver, and dissolving the oxidized metallic silver and undeveloped silver halide with a fixing agent. The oxidation of metallic silver is achieved with an oxidizing agent, commonly referred to as a bleaching agent. The dissolution of oxidized silver and undeveloped silver halide can be accomplished concurrently with the bleaching operation in a bleach-fix process using an aqueous bleach-fix solution that contains both a bleaching agent and a fixing agent, or subsequent to the bleaching operation by using a separate fixer bath. For simplicity, hereinafter, we refer to both bleach-fix solutions and fixer baths as fixer baths.
It is highly desirable to process a photographic recording material as rapidly as feasible. In particular, keeping the silver removal steps, which consume a large amount of the total process time, as short as possible, is an attractive manner in which to shorten the overall processing time. Juxtaposed to the desire for a rapid process is the desire for, and the need for, photographic recording materials and processing solutions that require lower chemical usage and that generate less polluting chemical waste. One way to reduce chemical waste is to use lower replenishment or regeneration rates for the processing solutions, and reduce the volume of solution that overflows to the waste stream.
Unfortunately as fixer baths are used, reaction products accumulate in the solutions. These products, mainly dissolved silver and halide ions, retard the fixing reaction and impair the performance of the fixer bath. Iodide ions, if present, have a very strong retarding effect on the fixing process. The concentrations of silver and halide ions can become even higher, and their retarding effect on fixing become even more severe, as replenishment or regeneration rates for the processing solutions are decreased in an attempt to reuse or recycle more of the processing solution and thereby to decrease waste processing effluent.
Some degree of fixing improvement and waste reduction can be achieved by removing silver from used, or so-called seasoned, fixer baths by chemical and electrochemical means. But these treatments do not remove the detrimental halide ions, in particular, the especially harmful iodide ion if present, from the fixing solution, and so fixing performance cannot be completely restored and eventually the fixer solution must be discarded or replenished with more fresh solution.
Therefore, if iodide ion could be removed, it would result in a more rapid photographic process, and it would extend the life of the fixer bath, while enabling low replenishment rates to be used.
The removal of iodide ion from a fixer bath is made difficult by the presence of other solution components, such as thiosulfate, sulfite, and argentothiosulfate complex ions. It is desired not to remove the sulfite ions and the thiosulfate ion, which is the active fixing agent. Silver can be removed separately by means of several known methods. Unfortunately, many methods which might remove iodide ion such as oxidation, precipitation, complexation and ion exchange will be interfered with by these other anions. Sulfite and thiosulfate are easily oxidized. Many substances which precipitate or complex with iodide also react with thiosulfate. Anion exchange media will extract sulfite and thiosulfate, as well as iodide. Further, the problem of removing iodide is made more difficult by the high concentration of the potentially interfering components. The thiosulfate concentration is usually in the range of 0.1 to 2.0 molar. The sulfite-hydrogen sulfite concentration is 0.01 to 0.5 molar. Iodide concentration may be as high as 0.05 molar, but it is often desired to maintain it less than about 0.005 molar. Therefore, it is desired that the system intended for iodide removal should exhibit a selectivity for iodide over thiosulfate and sulfite.
U.S. Pat. No. 3,925,175--Fisch et al discloses removal of silver and halide by passing the fixing solution through a cathode chamber of an electrolysis cell. The electrolysis cell has an anionic semipermeable membrane separating the anode and cathode and further contains a solution of electro-active oxidizable species in the anode chamber. However, such semipermeable membranes are expensive and are often fouled or plugged by solution components making them ineffective for separating after a short time. Further, the process requires electrical equipment and power, increasing the cost and complexity of separation.
U.S. Pat. No. 4,313,808--Idemoto et al, U.S. Pat. No. 4,283,266--Hirai et al, and U.S. Pat. No. 4,207,157--Hirai et al disclose electrodialysis systems utilized with photographic developers. However, these systems too are prone to membrane fouling and require expensive electrical equipment.
European Patent Application 0 348 532--Ueda et al discloses contacting the fixing solution with an ion exchange resin to accelerate the fixing of a silver iodide containing photographic material and reduce the amount of waste fixing solution. However, such resins remove ions other than iodide ion, such as thiosulfate, sulfite, and complexed silver ion, as described above. This method is not necessarily successful in removing iodide from solutions which contain many other anionic components.
U.S. Pat. No. 4,948,711--Kojima et al discloses bleach-fixing and fixing solutions, containing dispersions (latexes) of cationic polymers or water soluble cationic polymers, that exhibit more rapid desilvering. However, such polymers and dispersions may not be safe for the environment and may contaminate waste effluent from the process. They may also contaminate processed photographic materials.
Therefore, there remains a need for an effective, easy to use system for removal of iodide ions from fixer baths, without expensive equipment, or contaminating chemicals.